The present invention relates to a new and improved method of producing a nickel-base single crystal superalloy cast article.
Nickel base single crystal superalloy articles may be cast with a coarse gamma prime phase dispersed in a gamma phase matrix. In order to improve the morphology of the gamma prime matrix, the single crystal article is subjected to a solution heat treatment. During the solution heat treatment, the article is heated to a temperature which is slightly below the solidus temperature of the article for a sufficient period of time to cause the gamma prime phase to go into solution with the gamma phase. Thereafter, the gamma prime phase is reprecipitated from the gamma phase in refined form. One known method of casting a single crystal article with a coarse gamma prime phase and subsequently solution heat treating the article to form a refined gamma prime phase is disclosed in U.S. Pat. No. 4,209,348.
During the solution heat treatment of a single crystal article, recrystallization of a portion of the article tends to occur in areas where there is an excessive stress concentration, that is areas of high stored energy and dislocation densities. The recrystallization is a result of nucleation of crystals at these areas and substantial growth in the crystals during the solution heat treatment operation. The excessive stress concentration can be due to the entrapment at critical locations of parts of an article by a ceramic mold as it is being cast and/or by mechanical impacts against the cast article. Of course, the recrystallization of the article at areas of stress concentration destroys the single crystal characteristics of the article.